4. Proteins, Intracellular Transport And Membrane Transport

Question 1

Describe the primary structure of a protein.

Answer 1

Sequence of amino acids in an immediate post-translational polypeptide chain. It is maintained by peptide bonds.

Question 2

Describe the secondary structure of a protein.

Answer 2

Folding of the local regions of primary structure into either alpha-helix or beta-pleated sheets, stabilised by hydrogen bonding.

Question 3

Describe the tertiary structure of a protein.

Answer 3

Folding of the secondary protein into 3D shapes with either electrostatic attraction, Van der Waals attraction or hydrophobic clustering.

Question 4

Describe the quaternary structure of a protein.

Answer 4

The formation of a protein complex consisting of one or more polypeptide chains, e.g. haemoglobin.

Question 5

Name the mechanisms used in membrane transport.

Answer 5

1. Passive diffusion - driven by electrochemical gradient
2. Integrated membrane proteins - facilitated diffusion | primary active transport | secondary active transport (co-transporters/symporters & counter-transporters/antiporters)

Question 6

Types of transporters involved in facilitated diffusion

Answer 6

1. Non-gated channels - facilitate diffusion down the concentration gradient
2. Gated channels - has a gate, sensors and selectivity filters. More specific. (E.g. voltage-gated, mechanical, ligand-gated)
3. Uniporters - carrier mediated. One-directly only as the outer gate closes after the substance enters the channel. Easily saturated.

Question 7

Mechanism of secondary active transport

Answer 7

1. A solute moves down the concentration gradient through the secondary active transporter.
2. This movement provides kinetic energy to pump another solute against the concentration gradient - this can be in the same or opposite directions.
3. ATP is used by Na/K ATPase to restore the electrochemical gradient of the primary solute.

Question 8

Symporters/Co-transporters

Answer 8

Secondary active transport in which two or more substances are being transported in the same direction.

Question 9

Anti-porters/Counter-transporters

Answer 9

Secondary active transport in which two or more substances are being transported in opposite directions.

Question 10

Mechanism of primary active transport

Answer 10

1. Hydrolysis of ATP into ADP releases energy.
2. This energy causes a conformational change in the transporter protein, causing the solute to be pumped AGAINST the concentration gradient.
   In Na+/K+ pumps,
3. The bound phosphate dissociates and reverts the protein back into its original shape. This shape has a lower affinity for K+ ions so the 2 K+ ions dissociates.

Question 11

Types of active transporters

Answer 11

1. P-type ATPase
2. F ATPase
3. V-ATPase
4. ABC transporters

Question 12

Mechanism of ATP-gated CFTR channel

Answer 12

1. Hydrolysis of ATP to ADP releases energy
2. This energy is used to OPEN the channel.
3. The chloride ion diffuses down the electrochemical gradient (not against) out of the epithelial cell into the Airway Surface Liquid (ASL) and mucus.
4. Na+ follows passively, increasing the total electrolyte concentration in the mucus.
5. Water moves out of the cell via osmosis.

What happens in cystic fibrosis?
- The ASL and mucus becomes dehydrated as the CFTR channels are no longer working.
- This causes the mucociliary pathway to stop. (Why does it stop though?)
- Thick mucus builds up, where partial and bacteria can collect and potentially cause infections.

Question 13

Describe the major fluid compartments of the body and state their distributions.

Answer 13

Water = 60% of body weight
1. Intracellular fluid (ICF) - 40% of body weight
2. Extracellular fluid (ECF) - 20% of body weight
> Interstitial fluid - 80% of ECF
> Plasma volume - 20% of ECF
> Transcellular fluid - too small

Question 14

What contribute the most to solute concentration (osmolality)?

Answer 14

Electrolytes (ionic substances)

Question 15

Major electrolytes in ICF

Answer 15

K+ and Mg+

Question 16

Major electrolytes in ECF

Answer 16

Na+ Cl- HCO3-

Question 17

Difference in composition between interstitial fluid and plasma volume.

Answer 17

Plasma volume has a lot more proteins.